On-again couple Katy Perry and John Mayer seem to be going strong, but you can imagine things got pretty awkward when they ran into Katy's ex Russell Brand while dining in Beverly Hills.

The lovebirds were already seated when Russell entered the restaurant, and the server made the mistake of almost seating them right next to each other! "Katy's face was like 'Oh s--t!'" a source tellsUs Weekly, adding that the Wide Awake singer crouched under her table when her ex walked in.

Eventually, the server realized her mistake and led Russell to another table, but that doesn't mean he didn't sneak a peek at the couple on his way to the bathroom. "He looked at them through a window!" the witness says.

And while we're on the subject -- can you believe it's been a year since Russell filed for divorce from Katy?